This is an N64 development cartridge for music and sound effects development. These cartridges were targeted for musicians and sound engineers working for game studios during the N64 era.

The cartridge would interface to a IBM windows PC compatible computer, via the LPT parallel port. A sophisticated software package was included with these dev carts allowing musicians to compose various Midi music tracks or sound effect samples. Developers could also create custom sound samples for use with the Midi tracks being played. This essentially turned the N64 console into a midi sound sampler of sorts.

Developers could also test and preview sounds and music in real time through the N64 console connected to a CRT monitor.

You can watch an in-depth Hardware & Software DEMO I posted here on my YouTube channel. Link below: